(a). Field of the Invention
This invention relates to a communication system, and more particularly, to the frequency hopping transceiver of the communication system.
(b). Description of the Prior Arts
Frequency-hopping (FH) communication system is a commonly used technique. One of the examples is the multi-band orthogonal frequency division multiplexing (MB-OFDM) system. The MB-OFDM system hops between three adjacent sub-bands in the channel, as shown in FIG. 1. In FIG. 1, fc1, fc2, fc3 are the central frequencies of the three adjacent sub-bands. B stands for a symbol rate of the MB-OFDM system (i.e., the bandwidth of the OFDM signal). TOFDM stands for a symbol interval. The OFDM signal frequency-hops to another sub-band every symbol interval in accordance with a specific sequence.
In prior art, a frequency-hopping controller within a baseband circuit of the transmitter is used to control the frequency of a carrier signal generated by a carrier frequency synthesizer, and the baseband signal can be modulated according to the carrier frequency for transmission. In the baseband circuit of a conventional receiver, a packet detector is used to detect if a packet has been received. If the packet is detected, it will activate the frequency-hopping controller to sequentially output a frequency-hopping control signal to the carrier frequency synthesizer, and the carrier frequency synthesizer will then output the carrier signal. The received radio frequency (RF) signal will be demodulated according to the carrier frequency to produce the original baseband signal.
Please refer to FIG. 2. FIG. 2 is a block diagram of the conventional transmitter of the MB-OFDM system. A signal mapper 201 converts an input into a mapped signal. An inverse fast Fourier transform (IFFT) device 202 is used for converting the mapped signal into a time-domain signal. A guard interval (GI) adding circuit 203 is used for adding the time-domain signal with a guard interval and then generating a baseband OFDM signal. The baseband OFDM signal is converted into an analog signal by a parallel-to-serial converter (P/S) 204 and a digital-to-analogue converter (DAC) 205. The sampling frequency fs of the DAC 205 is
            f      s        ≥          1              T        OFDM              =  B
This analog signal will pass through a transmission filter 206, in which the cutoff frequency (marked as fcutoff) is B/2. A frequency-hopping controller 209 controls the frequency of the oscillating signal of a frequency synthesizer 208, and a mixer 207 modulates the analog signal according to the frequency of the oscillating signal to produce a RF signal. As shown in FIG. 1, the carrier frequency output by the frequency synthesizer 208 isfc=fc1, fc2, or fc3 
In which, fc2−fc1=B and fc3−fc2=B.
Please refer to FIG. 3, which shows a block diagram of the conventional receiver of the MB-OFDM system. The frequency of an oscillating signal of a frequency synthesizer 308 is first set at an initial frequency (one of fc1, fc2 and fc3) A mixer 307 is used for demodulating a received RF signal according to the frequency of the oscillating signal into a baseband signal that will then pass through a low pass filter (LPF) 306 with a cutoff frequency of B/2. After the filtered baseband signal is sampled by an analog-to-digital converter (ADC) 305, the sampled signal is monitored by a packet detector 310 to set appropriate frequency-hopping time points. The frequency synthesizer 308 outputs different carrier frequencies according to the appropriate frequency-hopping time point controlled by a frequency-hopping controller 309. On the other hand, the sampled signal from the ADC 305 passes through a GI removing circuit 303 and a serial-to-parallel converter (S/P) 304, and then enters a fast Fourier transform (FFT) device 302 to be converted into a frequency-domain signal. After the compensation of a channel compensation device 311, a signal-demapping circuit 301 is used to generate the originally transmitted signal.
However, due to the very short frequency-hopping time stipulated by the MB-OFDM system, the frequency-hopping mechanism needs a very fast reaction rate. Also, as shown in FIG. 4, when the previously described mechanism is used for frequency hopping, transient impairment may happen to damage the performance of the MB-OFDM system. In FIG. 4, we can see that TFH is relatively smaller than TGI and TFFT within a symbol interval TOFDM. This means the time provided for frequency hopping is shorter. Therefore, the transient impairment is easy to happen in the transmitting signal within the period of TFH.
Moreover, in the receiver as shown in FIG. 3, if the packet detector 310 unfortunately makes a mistake, or the frequency-hopping controller 309 makes a wrong decision, it is very possible to lose signal due to improper setting of the frequency-hopping time point. This will cause unrecoverable continuous mistakes and damage the performance of the MB-OFDM system seriously.